Temperature dependence of liquid lithium film formation and deuterium retention on hot W samples studied by LID-QMS. Implications for future fusion reactors

In this work, Li-filled 3D-printed porous tungsten samples were exposed to deuterium (D) plasma in Magnum-PSI with a wide ion flux from 4 × 1022 to 1.5 × 1024 m−2 s−1 and with a corresponding wide temperature range from below Li melting point (180.5 °C) to above Li deuteride (LiD) melting point (∼690 °C). The formation, decomposition and melting of LiD have been directly observed in the experiment via infra-red thermometry and visually post-mortem while still in vacuo, and correlated to the D retained content. The LiD formation was characterized by a solid precipitate layer formed on the surface with high emissivity (0.6–0.9) characterized by a blue or dark blue color after exposure. The melting of Li–LiD layer was found to occur close to the temperature predicted by Li–LiD phase diagram. In situ nuclear reaction analysis (NRA) was applied to perform the measurement of D retained in Li samples immediately after exposure without breaking the vacuum. D depth profiles were determined by NRA, in which the highest D concentration (15–45 at.%) was found in the top several micrometers and decreases with depth to low levels (<5%) within 5–30 μm. No pure LiD layer was found on the sample surfaces, however a D concentration close to 50 at.% was observed on a Li-D co-deposited layer on the clamping ring in some cases. The experiments also indicate that the D retained increases with increasing temperature until ∼500 °C. At temperatures beyond ∼500 °C the dissociation of LiD starts to dominate and the deuterium retention started to decrease. Overall, D retained fraction for all cases was found to be below ∼2%, which is significantly different from literatures where full uptake has been suggested. A 1D reaction–diffusion (RD) model based on D diffusion and chemical reactions with Li has been built. D depth profiles from the RD modelling can roughly match that from NRA measurement and a low D retained fraction below ∼2% was also indicated by the model. The model can also help explain the relationship between D retained and the surface temperature and fluence. After D plasma exposure, either helium or H plasma was utilized to remove the retained D in Li and both were proved to be effective and the removal efficiency can be as high as 96% above 420 °C.

Section: Introductionmentioning

confidence: 99%

Deuterium retention and removal in liquid lithium determined by in situ NRA in Magnum-PSI

Ou¹,

Arnoldbik²,

Li³

et al. 2022

“…The deuterium recycling coefficient at the various surfaces in experiment is very uncertain, with strong dependencies on the temperature and relative particle fluxes of the deuterium and lithium species [9,27,28]. Generally, it is expected that deuterium absorption above 400 • C-500 • C falls off rapidly due to thermal decomposition of lithium deuteride, though below this temperature deuterium absorption is high.…”

Section: Sensitivity To Deuterium Recyclingmentioning

confidence: 99%

The effect of gas injection location on a lithium vapor box divertor in NSTX-U

Emdee

Goldston

2023

The lithium vapor box divertor is a proposed divertor design that will minimize contamination of the upstream plasma in a fusion device, while also ensuring protection of the target. In this design lithium is evaporated near the target by high temperature lithium surfaces, dissipating the plasma heat flux. The lithium vapor box has been predicted via the fluid-kinetic code SOLPS-ITER to achieve low (nLi/ne∽0.05) upstream concentrations of lithium and low target heat fluxes. Here we compare two choices of deuterium gas puff location using SOLPS-ITER, the Private Flux Region (PFR) and the Common Flux Region (CFR), and find significant differences in the contamination level required to reach an acceptable target heat flux (defined here as qmax tar≦10 MW/m2). Deuterium gas puffing from the PFR is seen to better reduce upstream lithium contamination. The difference in puffing location is seen to cause changes in the upstream flow of lithium ions. The PFR puff, having better access to the separatrix, can better reduce the upstream-directed flow of lithium near the separatrix which is the primary source of contamination due to a large thermal force in this region. Puffing from the CFR, partially due to inefficacy at reducing separatrix lithium flow, has higher lithium concentration within the plasma. Solutions that reduce the heat flux to below 10 MW/m2 have a range of lithium concentrations between nLi/ne∽0.01-0.12 depending on puff intensity, location, evaporator temperature and recycling at the various plasma facing components. The efficacy of the puffs is tested for sensitivity to deuterium recycling coefficient at the target, evaporators, and main chamber walls. A CFR located puff is found to be more dependent on the recycling coefficients used than a PFR located puff. regardless of the set of recycling coefficients chosen, PFR puffing achieves lower lithium contamination than CFR puffing for a given heat flux.

“…(b) The retention of evaporated or plasma injected lithium on W at several temperatures was investigated in order to assess its impact on a fusion reactor operating with hot first wall. It was concluded [53] that an exponentially decaying retention as the first-wall temperature is increased takes place, with negligible D/Li ratios at T > 350 • C. On the other hand, D retention of liquid Sn and LiSn alloys was studied in DC glow discharges of D. Retention of H isotopes in the form of bubbles has been observed in liquid tin exposed to high D fluxes. This has been attributed to the low solubility and diffusivity of D into Sn.…”

Section: Power Exhaust: Liquid Metalsmentioning

confidence: 99%

Overview of the TJ-II stellarator research programme towards model validation in fusion plasmas

Hidalgo¹,

Ascasíbar²,

Alegre³

et al. 2022

Self Cite

TJ-II stellarator results on modelling and validation of plasma flow asymmetries due to on-surface potential variations, plasma fuelling physics, Alfvén eigenmodes (AEs) control and stability, the interplay between turbulence and neoclassical (NC) mechanisms and liquid metals are reported. Regarding the validation of the neoclassically predicted potential asymmetries, its impact on the radial electric field along the flux surface has been successfully validated against Doppler reflectometry measurements. Research on the physics and modelling of plasma core fuelling with pellets and tracer encapsulated solid pellet injection has shown that, although post-injection particle radial redistributions can be understood qualitatively from NC mechanisms, turbulence and fluctuations are strongly affected during the ablation process. Advanced analysis tools based on transfer entropy have shown that radial electric fields do not only affect the radial turbulence correlation length but are also capable of reducing the propagation of turbulence from the edge into the scrape-off layer. Direct experimental observation of long range correlated structures show that zonal flow structures are ubiquitous in the whole plasma cross-section in the TJ-II stellarator. Alfvénic activity control strategies using ECRH and ECCD as well as the relation between zonal structures and AEs are reported. Finally, the behaviour of liquid metals exposed to hot and cold plasmas in a capillary porous system container was investigated.